In most pre-industrial societies infant care is a responsibility shared among all family members. In our post-industrial society with two working parents, infant care has again become a cooperative activity. We know little about the hormonal and behavioral mechanisms that underlie successful cooperative infant care. The cotton-top tamarin is a primate with a short generation time where all family members cooperate to care for new infants. We have developed non-invasive hormonal measurement techniques and a variety of observational and experimental behavioral methods that will be applied to a longitudinal study of ten family groups (five highly experienced, five newly formed) over three successive births. The goals are: 1.) to describe the hormonal changes that occur in both parents at the time of parturition and in the early weeks of infant caretaking and correlate these with behavioral observations of caretaking. 2.) to determine predictors of successful infant care. There is considerable variation in infant care success and the hormonal and behavioral characteristics of successful versus unsuccessful families will be determined. 3.) to determine the role of infants in regulating caretaking. Infants may be as important as caretakers in eliciting appropriate responses. The infant growth, sex, and behavioral style as potential influences on the amount and quality of caretaking received will be examined. 4.) to determine the role of early hormonal environment, sex, and behavioral style on infant development. Tamarins give birth to twins that share a placental blood supply. By examining mixed sex versus same sex twins, the role of intrauterine environment versus postnatal social environment on infant behavior will be determined. Experimental studies will be carried out to determine if male tamarins can detect the stage of pregnancy in their mates and in other females through olfactory cues and whether tamarins remember odors of relatives from whom they have separated for various periods of time. Finally, the applicant will examine whether the reproductive inhibition found in subordinate female tamarins is reversible and pursue further studies on the hormonal and social mechanisms influencing fertility inhibition. The results of these studies should provide an understanding of the social behavior and physiological controls of cooperative infant care, the role of helpers and how variation in behavior and physiology of parents and other helpers affects the quality of infant care. There are few studies that examine how infants influence the quality of caretaking received.